The invention relates to a particle beam system with a device for reducing the energy width of a particle beam.
In particle beam systems and especially in electron optical systems, it will be advantageous to increase the resolution of the system, in particular to reduce the spot diameter in low-voltage applications.
In low-voltage electron beam systems (beam energy below 5 keV), in particular in low-voltage secondary electron microscopes and related test and inspection systems, the system resolution which means the minimum achievable spot diameter D, is limited by the geometric spot diameter Dgeo, the chromatic aberration error Dchrom and the spherical aberration error Dsph of the objective lens involved:
D={square root over (D2geo+D2chrom+D2sph)}
The contribution of spherical aberration error Dsph can be neglected in low-voltage applications. The geometric spot diameter Dgeo is defined by the diameter of the particle beam source and by the source demagnification of the optics involved.
The chromatic aberration contribution Dchrom is determined by the following dependency:
Dchrom=Cchromxcex1xcex94E/E
with Cchrom: chromatic aberration coefficient of objective lens
xcex1: aperture angle of objective lens
E: primary beam energy
xcex94E: energy width of primary beam
In order to increase resolution or reduce probe diameter at low beam energy E,
1. Cchrom has to be made small, which can be achieved by good low-voltage lenses, such as single pole lenses or combined electrostatic-magnetic objective lenses or by correction means as described in EP-A-0 373 399,
2. xcex1 has to be reduced which, however, is limited by the diffraction aberration which increases with decreasing xcex1 and
3. xcex94E has to be reduced. This is normally done by applying electron sources with low energy width, e.g. cold and thermal field emission cathodes or photocathodes.
In order to reduce the spot diameter D (which means Dchrom) further, devices have to be provided in the particle beam system which reduce xcex94E of the primary beam.
In commercial low-voltage secondary electron microscopes and related equipment, no devices for reducing energy width have been realized until now, in spite of the fact that solutions are widely discussed. JP-A-1 264 149 relates to a charged particle beam system according to the preamble to claim 1. In the embodiment with merely two filters, the particle beam is deflected in that the beam seems to come from the center of the aperture arranged between the two filters.
Basically, a large number of monochromators and energy filters are known which are applied in transmission electron microscopes. An overview of such filters can be seen in: L. Reimer, xe2x80x9cEnergy-filtering transmission electron microscopyxe2x80x9d, Springer Series in Optical Sciences, Vol. 71, 1994.
One of those energy filters mentioned is a Wien filter, which consists of crossed electrostatic and magnetic deflection fields perpendicular to the optical axis of the particle beam system (see FIG. 1 and FIG. 2).
FIG. 3 demonstrates the effect of a Wien filter 1 on a primary beam 2 generated by a source 3. Both the electrostatic and the magnetic field of the Wien filter 1 will deflect the particle beam. In the case where the electrostatic and magnetic fields will act in opposite directions, the primary beam (in first approximation) will not be deflected. This condition, however, is only true for one beam energy E0. For a beam having an energy width xcex94E, the condition does not hold and the beam disperses as shown in FIG. 3. The degree of deflection of each particle will depend on its energy. By placing an aperture 4 at some distance from the Wien filter, only a certain energy range of the beam can pass. The other energies are stopped at the aperture 4. By this effect, the energy width of the primary particle beam 2 can be reduced or, in other words, the beam can be monochromated.
However, since energy dispersion is performed by beam deflection, not only an angular spread according to beam energy is performed, but also the image of the electron source is enlarged (see FIG. 4). Each non-matching energy seems to have a different off-axial image location.
This can only be avoided if an intermediate image of the source 3 is located in the center of the Wien filter as shown in FIG. 5. In this case, energy-dependent image enlargement can be avoided. Such an arrangement, however, needs a cross-over in the center of the Wien filter. However, each cross-over increases the electron-electron interaction which increases spot diameter especially in low-voltage systems by two effects:
1. Increasing the energy width of the primary beam, which happens in the beam path mainly in the cross-over,
2. increasing the spot diameter by electron repulsion during the total beam path.
Accordingly, low-voltage systems should avoid or minimize the number of cross-overs and must be as short as possible. According to these design rules, the device for reducing the energy width should avoid additional cross-overs and should not increase the length of the optical system.
Therefore, the best location of the Wien filter would be the source of the primary beam itself, which, however, is not practicable, especially in those cases in which a field emission source is used. Such cathodes have no real but a virtual source position, which is located inside the corpus of the emitter and which is, therefore, not accessible.
The object of the invention is to provide a device for reducing the energy width in a particle beam system without having drawbacks on the system resolution caused by other aberrations or limitations.
The two Wien filters have electrostatic and magnetic field directions acting on the particles in opposite deflection directions. Correspondingly, those particles which are deflected in the 1st Wien filter to the left are deflected in the 2nd Wien filter to the right and vice versa. The field strength of the 2nd Wien filter is excited in such a way that all particles seem to come from the source. Such an arrangement has the same functionality as one single Wien filter in the real or virtual source position.